Патент USA US3034877код для вставки
i at tet "is 3,034,870 Patented May 15, 1962 1 2 3,034,870 It is accordingly a primary object of this invention to provide an economical and ef?cient method for drying electrolytes suitable for use in the fusion electrolysis of METHOD OF REMGVENG AIR AND WATER CON TAMENANTS TRAPPED WITH Tl-E CRYSTAL STRUCTURE OF SODIUM CHLORHDE these metals. Other objects are to provide a more effi cient method for drying alkali and alkaline earth metal halides prior to their use as components in electrolytic bide Corporation, a corporation of New York baths from which reactive metals are to be deposited, No Drawing. Filed Dec. 24, 1958, §er. No. 782,617 and to provide alkali and alkaline earth metal halides 6 Claims. (Cl. 23-293) having a sufficiently low elemental oxygen and nitrogen This invention relates to a process for removing the last 10 content to permit the deposition of reactive metal of ex traces of air and water from salts used as constituents of ceptional purity from an electrolytic bath of such salts. electrolytic baths for the production of reactive refractory The above objects are achieved by heating the elec metals of groups IV, V, and VI of the periodic table. trolytic salt to a temperature between its melting point ‘One commonly employed method of producing reactive and boiling point in an environment free of deleterious refractory metals such as titanium, zirconium, and the contaminants such as oxygen, nitrogen, and water and like is by electrolysis of the metallic compound dissolved maintaining said evironment at a pressure not exceeding in an electrolyte consisting of fused alkali or alkaline atmospheric pressure during the heating period to purify earth metal halides. Electrolytes commonly utilized in said salt and render it substantially free of said Water and this connection are calcium chloride, mixtures of calcium air constituents. and magnesium chlorides, mixtures of calcium chloride, 20 In order to achieve optimum results, the salt may be Ralph M. Starla, Ramsey, NJ, assignor to Union Car sodium chloride, and potassium chloride, as well as so dium and potassium chlorides, either alone or in com bination. preheated for a length of time at a relatively low tem perature to remove some of the contained water prior to the more complete removal described above. This heat Major dif?culties in the production of reactive refrac ing step may be accomplished either under vacuum or at tory metals arise due to their easy contamination by 25 atmospheric pressure. A standard drying oven and a oxygen, moisture, and nitrogen introduced into the elec temperature of about 100° C. may be used, for example. trolytic cells with the electrolyte, either as air voids or One of the advantages of this invention is the fact that entrapped with mother liquor in the crystallized salt. Of the process is quite ?exible insofar as temperature, pres these impurities, oxygen is perhaps the most deleterious sure, and environment are concerned. The temperature since its presence renders the subsequent puri?cation of must be above that required to melt the salt and below the metal very di?icult and, in some cases, virtually im its boiling point. The lower the absolute pressure, the possible. A water content as low as 0.2 percent of the Weight of a particular electrolytic salt, a content actually quite low when one considers the hygroscopic nature of some compounds found satisfactory as major constituents of electrolytic baths (for example, calcium chloride), is sul?cient to prevent the production of ductile, high-purity metal in large crystal form, as is required by increasingly more rigorous metallurgical standards. Nitrogen and nitrogen~bearing contaminants are known to exert an em more thorough the removal of contaminants. However, if the surface of the salt is ?ushed with an inert gas, the pressure may be as high as atmospheric with satisfactory results. The time required for the removal of contaminants depends upon a great number of variables. For any particular case, the point at which the purging is substan tially complete may be determined by observing the vapor 40 pressure, in the case of vacuum heating, or by measuring brittling effect on refractory metals. the dew point of the e?luent gas, in the case of an inert gas flush. For example, if a salt is heated through its been suggested and tried. Efforts to eliminate the occlu melting point while contained in a vacuum of approxi sion of water and air by modi?ed recrystallization proce mately 25 microns of mercury, absolute pressure, the re 45 dures ‘were unsuccessful. In this connection, attempts lease of Water, air and other contaminants formerly en Several attempts to overcome these di?iculties have were made to produce monolithic crystals by decreasing trapped in the crystal structure will be followed by an solubility through the addition of acid or methanol, and increase in pressure-to 500 microns, for example. As by the slow evaporation of the salt solution under vac the contaminants are removed by the vacuum pump, the uum. In allcases the products obtained had microscopic pressure will slowly return to a value approximating the 50 original. At that point, the puri?cation step is substan internal chambers containing either air or brine. It has also been proposed to dry electrolytic salts at tially complete. 200° C. at atmospheric or reduced pressure. If the total Similarly, if the dew point of an e?luent inert gas is volume of the electrolyte is quite small, the amount of measured, a parallel phenomenon will be observed whereby water remaining after the above treatment does not ap the dew point will increase to a high value and will then de 55 pear detrimental, but if the bath volume is increased by crease when removal is complete, often to values as low as as little as a weight factor of 2, the quality of the metal produced decreases very sharply. Completely inadequate results are more than Another which the obtained by this method with cells containing about 5 pounds of electrolyte. procedure involves a pre-electrolysis stage in cell is operated under reduced voltage for a considerable length of time, moisture being removed by electrolysis of the oxygen-bearing compounds, by absorp tion by the metal deposited during that period, and by the heat of the bath. This process requires the discard ing of the metal ?rst produced, and is, therefore, eco nomically impractical. —80° F. or —90° F. It will be understood that so many variables in?uence the actual removal times that precise limitations cannot be attached thereto. The particular vacuum pumping system employed, for example, as well as deposits in the piping, dryness of the inert gas source, and the type of chamber employed will all have some bearing on the time. In the following examples, times have been chosen based 65 on either dew point measurements or pressure measure ments. Several modi?cations of the basic invention are pos sible. In one embodiment, the electrolyte material is main There is a great need for an ef?cient and cheap method tained at an absolute pressure not exceeding 300 microns which will remove the contaminants and thereby eliminate 70 for a period of 4 to 6 hours, the temperature being held their deleterious effects on the reactive metals produced at 50° C. to 100° C. above the melting point of the mate by fusion electrolysis. rial treated. 3,034,870 3 An alternative method consists in initially heating the material to a temperature slightly below its melting point ll specimen of the metal obtained using this electrolyte was 99 Brinell. Example III in an atmosphere of inert gas at a pressure not greater than 3 centimeters of mercury, and preferably not exceed ing 100 microns; subsequently increasing the temperature slightly over the melting point of the treated material, and adjusting the pressure to a value not exceeding 3000 microns, preferably while ?owing through the heating About 17 pounds of reagent grade sodium chloride were dried for 23 hours at 200° C. at a pressure of 3 centimeters of mercury. It was then heated in an en closed chamber to a temperature between 800° C. and 850° C. for a period of 14 hours. Approximately 8400 chamber dry, inert gas at a rate of about 3 to 5 liters liters of argon were passed through the chamber at atmo per minute. 10 spheric pressure during this period, and, based on dew ‘A further modi?cation of the invention is to ?rst heat point measurements, 16 grams of water or 0.2 percent the electrolytic material or salt under vacuum at a tem of the original salt weight were removed. The salt thus perature well below its melting point, and subsequently treated was used in an electrolytic bath for the produc to heat the material to a temperature slightly above its of tantalum metal, which exhibited a hardness of 170 melting point at a pressure not exceeding atmospheric in 15 tion Brinell. the presence of an inert gas, passing such gas through a A similar quantity of reagent grade sodium chloride melting chamber at a flow rate of 8 to 12 liters per minute. was also conventionally dried for 24 hours at a tempera In accordance with this variation of the invention, the salt may be heated even at atmospheric pressure. The ?rst two modi?cations of the invention will re move moisture at a faster rate than is the case with the third procedure. Investigation has shown that both vac uum treatments will satisfactorily remove moisture in a period of 5 to 7 hours, and that 13 to 15 hours are required ture of 200° C. under a pressure of 3 centimeters of mer cury. An electrolytic bath for the production of tan 20 talum was prepared from this salt, and the metal de posited therefrom was found to have a hardness of 253 Brinell, a value much higher than that exhibited by the metal produced from electrolytes treated according to method of this invention. for the third method, when treating about 17 pounds of 25 theAnother important effect achieved by this purification sodium chloride. In the three methods, the length of time procedure, and which is believed to be directly related of treatment is proportional to the quantity of material to the reduction of the impurity content of the metal was being puri?ed, and to the ?ow rate of the inert gas. the increase in crystal size of the deposited metal. Metal Close control of temperature is required to reduce volati obtained by the use of impure electrolytes is ?nely di lization of the salt with consequent risk of condensation 30 vided and powdery, whereas the metal produced from an within, and plugging of any associated piping. Any inert electrolyte puri?ed according to the method of this in monatomic gas such as helium, argon or neon may be vention is in the form of dendritic crystals, which are used. usually from 1 to 2 millimeters in diameter and 4 to 5 The instant invention has been successfully applied to a in length. This increase in crystal size is considerable number of electrolytes, and the following 35 millimeters desirable since large crystals are not as affected by atmos speci?c examples are given for a clearer understanding of pheric oxidation, and may be remelted to secure massive the invention, and to illustrate the principles and broad metal more easily than the ?nely divided material. application of its methods. As is well known, the hardness Another signi?cant advantage possessed by the larger of reactive refractory metals is sharply increased by the crystal form is the greater ease and rapidity with which presence of small amounts of such impurities as oxygen, 40 spent electrolyte can be leached from the metal. Such nitrogen, carbon, and hydrogen, so that hardness values leaching may be quickly and simply accomplished with are generally used as an indication of metal purity. Ac hot water without danger of deleterious surface oxida cordingly, hardness values will be given in these examples tion. The need for acids or special chemicals to prevent for later comparison with that of the metal obtained from hydrolysis and surface oxidation as required in the case salt treated according to prior art. 45 of powdery metal is accordingly eliminated. Example 1 Approximately 17 pounds of reagent grade sodium chloride were pretreated by vacuum drying for 23 hours The operation of the cell is improved by greater salt purity. The cell atmosphere is clear, and less salt than usual is transported from the bath by vaporization and entrainment in the etliuent gases. The e?iciency of cur at 200° C. and an absolute pressure of 3 cm. mercury. It 50 rent utilization and the metal recovery are both increased was then heated under pressure not exceeding 300 microns, and usually maintained near 100 microns, within the tem perature range of 850° C. to 900° C. for 6 hours. No inert gas was passed through the treatment chamber during this since there is no waste of energy through the electrolysis used as an electrolyte in the production of tantalum metal the fusion electrolysis production of reactive refractory of oxygen-‘bearing compounds. From the foregoing it will be apparent that the in vention provides a new method for purifying electrolytes treatment. The salt so treated was used as an electrolyte 55 and improves upon prior methods which have been tech in the deposition of tantalum metal. The metal thus ob nically and economically impractical. The electrolytes tained was found to have a hardness of 77 Brinell. so produced are free of the least traces of air and water, and their use in electrolytic baths for the production of Example II reactive metals results in a very pure, highly ductile prod Seventeen pounds of reagent grade sodium chloride were 60 uct having large crystal size. vacuum dried for a period of 23 hours at 200° C., and a As used in this speci?cation, the term “periodic table” pressure of 3 centimeters of mercury. Following this refers to a periodic chart of the type found at pages 56 treatment the salt was heated to 770° C. under a pressure and 57 of Lange’s Handbook of Chemistry, eighth edi of 25 to 50 microns, and when the temperature had be tion, 1952, published by Handbook Publishers, Inc. This is a continuation-in-part of my copending appli come equalized throughout the mass, the cell pressure was 65 allowed to increase to 2000 microns by regulating the addi cation Serial No. 427,886, ?led May 5, 1954, and en titled “Improvement in Process for Fusion Electrolysis tion of argon to a flow rate of 4 liters per minute. The of Reactive Metals,” and now abandoned. temperature was then raised to 825 to 850° C., and main What is claimed is: tained within this range for a period of 6 hours. The dew 70 point of the e?luent gas issuing from the treating chamber l. A method of removing water and air constituents entrapped within the crystalline structure of sodium was initially 65° F., but decreased to —20° F. by the end chloride intended for use as electrolyte components in of the treatment. The salt so treated was collected and by electrolysis. The hardness of a 100 gram arc-melted 75 metals of groups IV, V, and VI of the periodic table 3,034,870 6 which comprises heating said halide to a temperature between its melting point and boiling point in an en vironment free of deleterious contaminants, and main halide to a temperature between its melting point and taining said environment at a pressure not exceeding at~ mosphen'c pressure during the heating period to purify said halide and render it substantially free of said water and air constituents. 2. A method of removing water and air constituents entrapped within the crystalline structure of sodium chlo ride intended for use as electrolyte components in the 10 fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table which comprises heating said halide to a temperature between its melting point and boiling point in an en boiling point in an environment free of deleterious con taminants, and maintaining said environment at a pres sure not exceeding atmospheric pressure during the heat ing period to remove the remaining water and air. 5. A method of removing Water and air constituents entrapped within the crystalline structure of sodium chlo ride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory ,metals of 1groups IV, V, and» VI of the periodic table which comprises initially heating said electrolyte com ponent to a temperature below its'melting point at a pressure not exceeding 3 centimeters of mercury, in creasing the temperature above said melting point, de vironment free of deleterious contaminants, maintaining 15 creasing the pressure to a value not exceeding 3000 mi said environment at a pressure not exceeding atmospheric crons, passing a dry, inert gas over said electrolyte com pressure during the heating period, and ?owing a dry, in ponent to purify said electrolyte component and render ert gas across the surface of said halide to purify said - it substantially free of Water and air constituents. halide and render it substantially free of said water and 6. A method of removing water and air constituents 20 entrapped within the crystalline structure of sodium chlo air constituents. 3. A method of removing water and air constituents ride intended for use as electrolyte components in the entrapped within the crystalline structure of sodium chlo fusion electrolysis production of reactive refractory met ride intended for use as electrolyte components in the fu als of groups IV, V, and VI of the periodic table which sion electrolysis production of reactive refractory metals comprises placing said electrolyte ‘component in a heat of groups IV, V, and VI of the periodic table whichicom 25 ing chamber, reducing the pressure to a value not ex prises heating said halide to a temperature between its ceeding 300 microns, heating said electrolyte component melting point and boiling point in an environment free at a temperature 50° to 100° C. above its melting point until measurements indicate substantially no e?lux of of deleterious contaminants, maintaining said environ ment at a pressure not'exceeding atmospheric pressure moisture, and cooling said electrolyte component where during the heating period, and ?owing a dry, inert gas 30 by the puri?ed electrolyte component is obtained in the selected from the group consisting of helium, argon, and solid state. neon across the surface of said halide to purify said halide and render it substantially free of said water and air constituents. 4. A method of removing water and air contaminants 35 entrapped within the crystalline structure of sodium ‘chlo ride intended for use as electrolyte components in the fusion electrolysis production of reactive refractory metals of groups IV, V, and VI of the periodic table which comprises initially heating said halide to a tem 40 "t"a».at References Cited in the ?le of this patent UNITED STATES PATENTS 2,564,498 Nisbet ______________ __ Aug. 14, 1951 2,762,684 Wainer ______________ __ Sept. 11, 1956 OTHER REFERENCES Mellor’s “Modern Inorganic Chemistry," 1939, Long perature below its melting point and at a pressure not ex ceeding atmospheric pressure to remove substantial mans, Green and Co., N.Y., pp. 160-161. Kroll et al.: “Bureau of Mines Report of Investiga amounts of water and air therefrom, further heating said tion #4915.” I '